Flame ionization detection (FID) was originally developed for use in conjunction with gas chromatography and is nominally designed to operate at relatively low mobile phase flow rates (i.e., up to around 40 mL/min). However, FID is now also commonly employed in conjunction with compressible fluid-based chromatography, hereinafter referred to as “CFC,” in which the mobile phase, typically compressed carbon dioxide, is used. An example of CFC is supercritical fluid chromatography, hereinafter referred to as “SFC,” in which carbon dioxide in its supercritical state or near supercritical state is typically used as the mobile phase. When decompressed, the CFC mobile phase achieves much higher flow rates than that of gas chromatography. When interfacing a CFC system to an FID detector, a transfer line connected to the CFC system transports some or all of the mobile phase flow to the detector. Due to the compressed nature of the mobile phase, this transfer line must also function as a flow restrictor (i.e., to maintain system pressure). The compressed mobile phase enters the restrictor as a dense fluid and exits as a decompressed gas. Since the fluid expands as it transitions to a gas, the volumetric flow rate at the outlet of the restrictor is considerable. As a result of this expansion, precise positioning of the end of the restrictor within the FID burner is required to ensure stable flame operation and optimal analyte response.
Conventional restrictors in FID burner assemblies are configured such that the flow stream exits the restrictor and into the burner in a direction substantially parallel to the longitudinal axis of the burner. This configuration can be accomplished by simply cutting the end of the restrictor at an angle perpendicular to its longitudinal axis (i.e., a “square cut” restrictor) and is done for ease of manufacture and to ensure restrictor-to-restrictor reproducibility. However, this restrictor design requires considerable burner length to allow for the mobile phase to fully decompress and slow in linear velocity so as to both maintain a stable flame and achieve an optimal analyte response. Thus, the use of “square cut” restrictors in FID burners results in a narrow window of distance from the flame in which the position of the restrictor must be precisely optimized. In a worst case scenario, the mobile phase flow rate out of the restrictor may be so great that the FID burner may not be long enough for optimal positioning of the restrictor at all. A further complication encountered when using such restrictors is that its position within the FID burner must be re-optimized when any change in restrictor flow rate is experienced, such as when the system pressure is changed (i.e., density programmed separations) while operating a split-flow interface to the FID or when the mobile phase flow rate is changed while employing a full-flow interface (i.e., changing column diameter).
Thus, there exists a need for improved FID burner assemblies that do not require precise positioning and re-positioning of the restrictor in order to optimize analyte response and which provide for enhanced flame stability during operation.